If you downgrade your Databox subscription to a plan that does not include Metric Builder, you will be prompted to delete any Custom Metrics previously created in your Account.
If the Custom Metrics were not created directly in your Account, they will continue to actively sync data. However, the Custom Metrics can not be edited from within your Account. This would be the case if the Custom Metric was created from a public Template or from a Databoard that was copied from another Databox Account (i.e., Agency Account to Client Account).